On the fsfs-format7 branch:  Teach pack code to prevent items from
overlapping block boundaries.

This significantly improves speed in particular for parsed items where
line-sized prefeches may cause APR files to jump back and forth between
blocks.  It also allows for many blocks to be read only once instead of
2 or 3 times due to overlapping items "pulling them in" again.

The optimization trades < 1% of size for ~30% savings in data transfer
depending on scenario and items sizes.

* subversion/libsvn_fs_fs/pack.c
  (write_null_bytes): new utility
  (copy_items_from_temp): prevent block boundary crossings where feasible;
   add docstrings

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/fsfs-format7@1446100 13f79535-47bb-0310-9956-ffa450edef68